In general, a magnetic recording medium is produced by applying a magnetic coating solution containing a ferromagnetic powder and a binder on a non-magnetic support (hereinafter, occasionally referred simply to as "support") to form a magnetic layer, conducting orientation etc., drying the magnetic layer, and then conducting a calendering treatment so as to improve the surface smoothness, etc., of the magnetic layer.
A magnetic coating solution is usually prepared by kneading a binder, a dispersing agent, a lubricant, etc., using an organic solvent. In recent years, with the trend toward high-density magnetic recording, magnetic recording media of this kind also are required to have highly improved durability, and for this reason, the binder compositions used have changed from conventional non-hardenable types to hardenable types.
In the case where a hardenable binder is used, a magnetic coating solution has been prepared by a method in which at least one of the ingredients for the hardenable binder or a hardening catalyst is kneaded into the coating solution at the final stage of the production process, in order to prevent a hardening reaction during the production of the magnetic coating solution. The magnetic coating solution obtained after the dispersing step should be applied on a flexible support as soon as possible because a hardenable ingredient or a catalyst has already been added to the solution. Since the hardening reaction of the binder can proceed in a magnetic coating solution allowed to stand for a long period of time, magnetic recording media obtained by coating such a coating solution are not satisfactory in properties that high-density recording media are required to have, i.e., squareness ratio, surface properties, durability, etc. In conducting surface treatments such as, particularly, a calendering treatment, whether or not the treatments provide good effects depends on the degree of the progress of the hardening reaction of the magnetic layer. In connection with such a problem, JP-B-58-10073 (The term " JP-B" as used herein means an "examined Japanese patent publication") discloses a method in which a magnetic coating solution is applied on a support while continuously adding a hardening agent to the magnetic coating solution, thereby to eliminate a disadvantage where a hardenable binder is used.
As described above, previously the formulation or preparation of conventional magnetic coating solutions has been done by paying attention only to the reaction of the hardening agents. To take an extreme example, alcoholic - OH groups, water, etc., reactive to hardening agents have also been controlled during the production steps. However, as a result of recent studies and development by the present inventors, it has been found that consideration should be given not only to the hardening agents but also to the preparation of other ingredients and to the calendering treatment used for smoothing the surfaces of magnetic layers, for the purpose of improving the quality (Y.multidot.S/N, adhesion, value during running), etc., of coating films of magnetic recording media and also for attaining uniform and stable production. For example, JP-A-60-147931 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") addressed problems where a ferromagnetic powder, for example, ferromagnetic metal powder, having a large magnetic moment and which is advantageous for high density should be dispersed for a long period of time because of its poor dispersibility into a binder. Hence, such a process is industrially disadvantageous, and even if the powder can be dispersed, this dispersion (magnetic coating solution) will results in agglomerates if it is left standing for a long period of time.
In order to overcome such problems, a method has been proposed in which ingredients not including a lubricant are treated at the time of kneading to prepare a magnetic coating solution and, just before the above-described magnetic coating solution is applied on a non-magnetic support, a lubricant is added and mixed, as composed with conventional methods where a ferromagnetic powder, a binder, a lubricant, an abrasive agent, etc., are kneaded all together to prepare a magnetic coating solution. Specifically, JP-A-60-147931 gives an example in its Examples that a hardening agent (triisocyanate) and a lubricant (fatty acid) are post-added, and squareness ratios and surface gloss have been greatly improved by this method. Conventionally, however, coating and calendering treatment have not been conducted successively in such a process, but a method has been employed in which coating is conducted, followed by heat drying and rolling up the medium, and thereafter a calendering treatment is conducted with heating again. However, this method cannot sufficiently satisfy desired properties such as brilliance signal (Y.multidot.S/N) and surface properties, which are now required for high-density recording.